Conventional transmission systems include a system that uses a transmission apparatus having a 1-to-n redundant configuration in which one station is connected to another station through normally used n working-system transmission paths and one standby-system transmission path. When a fault occurs on a working-system path (transmission path, unit, or line), the transmission apparatus switches from the working-system line (working line) to a standby-system line (protection line). An optical signal or electrical signal is transmitted through the transmission path.
1-to-1 or 1-to-n line switching carried out upon the occurrence of a fault is defined in the communication standard provided in GR-253 and ITU-T Standard G. 841. In line switching, a fault countermeasure command is transmitted and received between a monitoring device and the transmission apparatus, and 1-to-1 or 1-to-n line switching is carried out. Switching information is defined as such a command in K1 and K2 bytes included in the header of a transmission frame. When a fault occurs on the working system, the K1 and K2 bytes representing switching information are transmitted and received between one transmission apparatus and another transmission apparatus, using a frame transmitted through the standby-system line.
FIG. 29 is a diagram of a definition of K1 and K2 bytes in conventional line switching. In the K1 byte, four bits (bits 1-4) define a switching request, while the other four bits (bits 5-8) define a switch request channel (switch request channel number). In the K2 byte, four bits (bits 1-4) define information of a channel switched to a standby-system channel (bridging channel number), one bit (bit 5) defines the line switching method (architecture), and three bits (bits 6-8) define the mode.
In the diagram, switching requests defined by the bits 1-4 of the K1 byte are listed in descending order. Line switching is thus carried out according to the switching request highest in priority, i.e., highest on the list. For example, Forced Switch represented by the value of the bits 1-4 “1110” is higher in priority than Manual Switch represented by “1000”.
Among conventional techniques for carrying out line switching according to such a definition is a technique in which a standby-system line is allocated dynamically in a standby system in a 1-to-n redundant configuration to prevent instantaneous disconnection that occurs in a switchback process ensuing restoration from a fault to enable long hours of operation using a standby-system transmission path. The technique provides a configuration such that a standby-system line is not established statically upon the occurrence of a fault and such that a transmission path used as a standby-system path is determined dynamically based on the quality and priority of a transmission path at the time of the occurrence of a fault and restoration from the fault (see, e.g., Japanese Patent Application Laid-Open Publication No. 2001-339370).
FIG. 30 is a diagram of state transition that is made in response to a switching request based on a conventional definition. FIG. 30 depicts transition between different states, with attention being focused on switching in slots (units or lines) of the station. For convenience, only some of defined states of FIG. 29 are depicted in FIG. 30. The depicted states include No Request P1, Manual Switch P2, Signal Fail (SF) P3, and Forced Switch P4. Signal Degrade (SD), etc., is also included as a defined state.
In a state of Manual Switch P2, redundant path switching is executed automatically, irrespective of a line switching instruction by a maintenance person when a path error rate exceeds a threshold for the bit error rate specified for a state of SD or when the path enters a state of SF. In contrast, in a state of Forced Switch P4, even when the path enters the state of SD, where the path error rate is higher than the threshold for the bit error rate specified in the state of SD, or enters the state of SF, automatic redundant switching in response to such transition is not executed consequent to the Forced Switch having a higher priority than SD and SF (P4), as depicted in FIG. 29.
A problem arises, however, when removal or a fault of a unit of the transmission apparatus, or a line fault, such as the disconnection of a cable from the unit, occurs after transition to the state of Manual Switch P2 defined in FIG. 30. This is a situation, for example, where such a fault occurs when the maintenance person has set the unit to the state of Manual Switch for a given reason. This situation is equivalent to a condition for a state having a higher priority than the state of Manual Switch P2 (SF (High) P3 in FIG. 30), resulting in transition p2 (depicted in FIG. 30) to automatically clear the state of Manual Switch P2 and make transition to the state of SF (High) P3 (depicted in FIG. 30). This consequently means that the state of Manual Switch set by the maintenance person changes into another state without being noticed by the maintenance person.
After the occurrence of such a fault, when the maintenance person carries out restoration work, such as replacement of the unit, a Small Form Factor Pluggable (SFP), which is equivalent to a port and serves as an optical transmitting/receiving module mounted on the unit, an optical fiber, etc., and mounts or connects a new unit, SFP, cable, etc., the corresponding path abruptly switches back from a standby-system line to a working-system line. At this time, the maintenance person is not able to check the switchback or errors, leading to a problem in that if trouble occurs with a newly mounted unit or cable, signal disconnection occurs.
To prevent such a problem, the maintenance person may set the unit in the state of Forced Switch P4 rather than the state of Manual Switch. Although the state of Forced Switch P4 has a high priority, this state is incapable of relieving a different unit or line from a fault and may cause serious trouble if the maintenance person errantly forgets to clear the state of Forced Switch P4. It is desirable, therefore, for the state of Forced Switch P4 to not be used if possible. In this manner, dealing with a specific event may be difficult if the conventional definition of states alone is applied.